Just bought a 2002 Jetta TDI. Before I purchase the car from a wholesaler I called the VW dealer where the car was traded in to ask if there were any problems. They said the only problem that they found while taking trade was a clogged EGR, which they said they DID NOT fix.
Knowing this I figured I would have to clean it when I got it. So I bought the car, and have been driving for <1 week and the MIL came on showing a P0401 (EGR low flow). Knowing what I knew from before the sale this seemed logical.
Well last night I went to clean the EGR valve and found a couple wierd things. First off, one of the bolts that connects the EGR to the intake was only threaded into the housing about 1/2", meaning there was no way a vacuum was created, the bolt was tight because it was cross threaded, so I took it out cleaned it up and put it in right. Secondly, when I removed the EGR valve is was clean. At least the main pass through from the MAF to the intake was clean, no build-up. The plunger looked pretty dirty but then the intake from the EGR cooler looked clean too. The "flex" pipe from the cooler to the EGR had maybe 2-3 mils of build-up on it. After finding this I did not clean it, I put the system back together thinking it was the loose bolt causing the problem. Well, I pulled the negative on the battery "resetting" the computer but the light has remained on.
Another thing to note, I added deisel kleen to the second tank I ran through and that tank the light turned off, then the next tank it turned back on and hasnt gone off since.
The car runs fine, but has had a slight hiccup at start up. I'm unsure where to start. I'd like to try to figure this out myself, but if someone knows of a good mechanic in central iowa I am open ears, the dealer is a rip-off.
Check that the EGR valve vacuum diaphragm is not leaking. Just remove the vacuum line to it, put a short piece of vacuum hose on it and suck on the hose. If it holds vacuum, that's not the problem. If it has a vacuum leak, replace the EGR valve. Here is a great place to get them 9and other stuff you may need for your TDI):
Or the EGR cooler may be plugged. You can rod out most of the cooler tubes using a coathanger wire, but the pipes to the cooler have to be removed. You can then blow out the soot with a can of Dust Off.
Update - 5/11/12 - Borrowing a OBD II from a buddy at work. Ran out at lunch and checked the code again is actually a P0402, not a P0401. Reset the codes, wondering if I didnt get the battery all the way drained when I found the loose bolt. Anyhow, would a 0402 make any difference in what i am looking for over a 0401? 02 being excessive flow, 01 being low flow???